Process of chemically treating and bleaching cellulose pulp



Patented May 17, 1932 UNITED STATES GEORGE A. RICHTER AND MILTON o. scnuR, or BERLIN, New nanrsnfmn, assIc-inioas To BROWN comm, or BERLIN, new HAMPSHIRE, A CORPORATION or MAINE PROCESS or cHEMIcALLY TREATING AND BLEAGHING ontnunosnrunr No Drawing. Application filed June 6,

This invention relates to the production of a bleached cellulose pulp of high cleanliness, whiteness, and low copper number. I

In our Patent 'No. 1,639,704, dated Au ust 23, 1927, it is disclosed that when free al ali is present during the bleaching of cellulose pulp, a product of high whiteness may be obtained without injury to the strength of the fibers and without lowering the alpha cellulose content of the, pulp through. the formation of ox'ycelluloses. The free alkali is preferablyadded to the bleach liquor at the start of bleaching and somewhat in excess of the amount necessary to'be gradually consumed 16 by reaction with acidic constituents initially present in the unbleached pulp or formed during bleaching, so that a distinctly alkaline condition is maintained throughout the bleaching operation. The alkali apparently 20 makes the reaction between the bleach and pulp more selective in so far as the removal of impurities from the pulp is concerned. It also appears to have a solvent effect upon such oxycelluloses as may be present in the pulp at the start of bleaching, or asmay be formed during bleaching. This patent further teaches the fact that higher temperature may be safely used during bleaching if free alkali be present in the bleach liquor, thereby elimimating the necessity for refrigerating the bleach liquor in hot weather, especially when stock is being bleached at a high density. Because of the protective influence which the alkali exercises over the cellulose, danger of local ove'rbleaching is avoided, so that raw stock and strong bleach liquor may be uniformly admixed even in the summer .When local overbleaching would ordinarily be certain to occur, particularly during initial mixing of the comparatively high temperature stock, resulting in a weakened stock and in a comparatively high bleach usage. A further advantage resulting from the use of free alkali during bleaching is the reduction in the number of resinous specks usually associated with the fibers, and accordingly the production of a cleaner pulp.

We have now found that if an alkali such as caustic soda is added tothe raw stock before the bleach liquor, and is allowed to remain 1928. Serial No. 283,452. g

in contact therewith at about room temperature for several hours, it has better opportunity thoroughly to penetrate the stock and to loosen resinouslspecks. This initial treatment with alkali, while advantageous when usingany type ofbleach, is ofmark'ed ad vantage when calcium base-bleach, such as calcium hypochlorite, is subsequently cemployed,for when the alkali and calcium hypochlorite are added simultaneously to the raw pulp, there is a tendency for the bleach to react with the resins to form insoluble resinates and oxidation products which become fixed on the pulp, thereby interfering to an appreciable extent with the solution or dispersion of'the resins, and the attainment of hi h whiteness. Even when sodium hypochlorite bleach issubs'equently used, the initial treatment with an alkali such as caustic soda is advantageous, in that the resins are peptized and brought into a condition more susceptible to attack and removal by the bleach. [The stock is preferably washed afterthe initial treatment with alkali, as resinous and other coloring substances are loosened thereby and may be readily removed. This not only permits subsequent bleaching tobe done with less bleach than usual, but makes possible the production of an unusually white and. clean pulp of high strength and low copper number.

The following are typical procedures, such as have been studied in the laboratory and carried out in the mill on a large scale.

Ewample I 'A raw or unbleached sulphite pulp prepared from s ruce chips as usual was treated as a suitably owable aqueouspulp suspension, for instance at a stock density of 11%, at a temperature of F. with%% caustic soda all chemicallpercentages are based on air-dry weight of the pulp used as a raw material). At the end of two hours, caustic soda had been consumed. To the pulp was then added 15% calcium hypochlorite bleach and caustic soda, and the mixture was al lowed to react for eight hours at about 80 F. At the end of this period, it was found that 14% bleach and a total of 1% caustic soda till till

(including that previousl added) had been consumed. The bleach washed, antichlored in sulphurous acid solution, and again washed. Although the resin content of the original stock was 0.64% and was 0.5% after bleaching as usual in one ste the resin content of the product ob-,

tained y following the procedure herein outlined was 0.30%. The fact that the initial treatment is effective in removing resins is clearly indicated by other observations. For.

4182.56. I Y Y Ewample II A stock similar to that used in the foregoing example was initially treated with alkali as therein described, was then washed, and treated for eight hours at a stock density of 11% and at a temperature of 80 F. with calcium hypochlorite bleach liquor containing 15% bleach and caustic soda. Practically all of the bleach and caustic soda was consumed. The bleached stock was then washed, antichlored, and a ain washed as in the foregoing example. t showed a material decrease in speck count and had a copper number approximately that of the product obtained in the foregoing example. The reduction in the speck count is quite significant, for the specks eliminated in this treatment are. of a resinous character, which are not removed by therifiiers in the mill which take out most of the other specks.

The stren h and tear resistance of samplesof the aten product obtained in both examples were substantially similar tothat of the product obtained by bleaching in the usual way, but the whiteness of the products obtained by processing according to the ex am les cited was markedly higher.

n a mill where a continuous process is being practised, about caustic soda may be added to the raw sulphite stock entering the system at 12% consistency and at about F. At a point in the system reached by the pulp about two hours later, about 12% to 15% bleach may be added and the tem erature raised to F. About o the caustic initially added will be consumed in the two-hour treatment before the bleach is added, but the stock will remain in a distmctly alkaline condition until practically all the bleach is consumed.

In addition to the advantages hereinbestock was then fore enumerated resulting from the application of the process of our invention, we have found that in actual mill practice we produce a final product of higher pH value than that associated with pulp bleached in the usual way. Thus, the raw .stock bleached in the usual way resulted in a dry sheet of pulp, 7

sulted in a dry sheet of pulp having a pH,

er pH value of about 4.8 to 5.5. The hi value connotes reater stability in t e pulp. Results obtained from largescale tests have shown that the stock of higher pH value will better endure a. long continued beating. When beaten, for exam le, together with rag stock, as is done in mills of limited beater capacity, the wood pulp fibers still have ood strength when the tough rag fibers have een properly hydrated.

The process of the present invention has especial utility when applied in the treatment of raw sulphite pulp to produce a product which may be hydrated or gelatinized as readily as the usual sulphite is suitable as for the manufgcture of hard, rattly papers of good strength and tear resistance. Our product is, moreover, superior in the important characteristics of whiteness and cleanliness. The initial treatment of the raw pulp with weak solutions of alkali at about room temperature, as herein described, does not change the character of the fiber as to its alpha cellulose or pentosan content to a pronounced degree, but is more especially effective in the removal, or in facilitating the subse uent removal by bleaching, of troublesome leach-resisting and coloring impurities whichotherwise require a more drastic ulp and which bleaching treatment and cause a product of 1 inferior-whiteness, cleanliness, and copper number, to result. Our process is practicable in that it maybe carried out with a low alkali and bleach consumption, without the use of heat, and without requiring equipment other than that usually employed in a bleaching process.

While in the specific examples given the alkalinity of the dilute solution at about room temperature used in the treatment of the pulp and in maintaining the bleach liquor alkaline is afforded by caustic soda, other chemicals, such as sodium carbonate, sodium phosphate, or borax may be used for this purpose.

Again. while the specific examples deal with ment. For instance, raw sulphite'stock may a be allowed to react with 1% caustic soda at a stock density of about and at about 70 F. for about four hours, whereupon it 5 may be washed to remove dark-colored, turbid reaction products. It may then be'subjected to a series of refining treatments, in-

cluding a treatment with chlorine, a washing treatment, alkaline digestion at high temperature, another washing treatment, a bleaching treatment, and a final washing treatment. If desired, the treatment with dilute alkali at room temperature may be performed after the chlorine treatment, or immediately before bleaching, rather than upon the raw stock. So, too, this treatment may be included in a refining process which involves the use of kraft pulp as a raw material and the bleaching of such pulp ina series of distinctly alkaline bleach liquors, the treatment being introduced before the series of bleaching is started or between the bleachings of the series. It may also be included as a preliminary step in a process which involves,

the bleaching of pulp in two steps, using chlorine water and hypochlorite bleach liquor in the order given, or vice versa, the bleach liquor preferably being maintained alkaline during the entire bleaching operation.

Having thus described certain modes of practising this invention, it is evident to those skilled in the art that various changes and modifications might be made therein without departing from the spirit 0r.scope of invention as defined in the appended claims.

What we claim is: 1. A process which comprises treating unbleached sulphite pulp at about room tern"- perature as a flowable aqueous pulp suspension in a solution containing not more than about to 1% caustic soda based on the weight of air-dry pulp, washing the pulp, and bleaching the pulp. 2. A process which comprises treating unbleached sulphite pulp at about room temperature as a flowable aqueous pulp suspension in a solution containing not more than about to 1% caustic soda based on the weight of air-dry pulp, washing the pulp, and w bleaching the pu p in a distinctly alkaline bleach liquor.

3. A process which comprises treating unbleached sulphite pulp at about room'temperature with a solution of caustic soda of such low alkalinity that practically no re finement of the pulp is efl'ected but much of its resin content loosened, washing the pulp matfree of solution and loosened reslnous ter, and bleaching the pulp.

4. A process which comprises treating unbleached wood pulp at about room temperature with a solution of alkali of such low' alkalinity that practically no refinement of the pulp is effected but much of its resin content loosened, and bleaching the pulp.

.5. A process which comprises treating unbleached wood pulp at about room temperature, with a solution of alkali 'o'f-such'low alkalinity that practically no refinement of the pulp is effected but much of its resin content loosened, and bleaching the pulp in a from the pulp while substantially preserving its alpha cellulose content, and then bleach ing the pulp.

A process which comprises treating un- A bleached wood ulp at about room temperatureas a flowa le aqueous pulp suspension in an alkaline solution whose alkali content does not exceed about 1% caustic soda equivalent based on the weight of dry pulp, thereby loosening impurities of the nature of resins from the pulp while substantially preserving its alpha cellulose content, and then 1 bleachmg the pulp in a calcium hyprochlorite bleach liquor.

In testimony whereof we have aiiixed our signatures.

' GEORGE A. RICHTER.

MILTON O. SCHUR.

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